670 ZOOLOGY. 
of early fishes. The earlier Batrachians were tailed, the 
tailless toads and frogs in general appearing last, as the 
tadpole precedes the frog condition. 
Extinction of Species.—The laws governing the extinction 
of animals are obscure, but we know that gevlogical extinc- 
tions must have been due to natural causes, since the earth 
has at different periods evidently undergone great changes, 
sufficient to account for the death of such species as were 
unable to withstand the oscillations and changes of climate. 
In Paleozoic times existed multitudes of animals which, judg- 
ing by their descendants of later times, belonged to old-fash- 
ioned, obsolete, useless types. They cumbered the ground, 
and were destroyed by the beneficent action of unerring natu- 
ral laws promoting the decay and extinction of antiquated 
forms, and the recreation, by the laws of transmission with 
modification, of new, improved types, useful in their day and 
generation as stepping-stones to a still higher, more improved 
stock. That the extinction was due to causes acting pri- 
marily from without, and secondarily from within by trans- 
mission force, seems demonstrated when we take into ac- 
count the destruction of life which we know took place 
during and at the close of the Glacial Period, when the 
earth was swept with glaciers, and afterward garnished 
with the vegetation and fresh life of the post-glacial times, 
and made ready for the abode of man. Thus the death of 
species by the action of laws that we can comprehend in- 
volves the recreation of new and improved animal forms by 
laws that we can at least in part, if not fully, understend, 
